Since their commercial introduction more than a decade ago, lithium ion secondary batteries has rapidly gained in their market share, a rate of adoption by consumers that is unseen in Cd—Ni, MH—Ni, and lead-acid batteries. This rapid development is a result of their exceptional characteristics that include: a high operating voltage, a high relative energy, wide operation temperature range, low self-discharge rate, long cycling life, low energy loss, zero memory effect, pollution-free operation, and improved safety. Lithium-ion batteries are widely used as the power source in small and expensive electronic devices such as cellular phones, laptop computers, medical devices, video cameras, and digital cameras. They have also gradually replaced traditional batteries in aerospace, aviation, naval, satellite, telecommunication, and military applications.
Most newer lithium-ion batteries use carbon material for their negative electrodes. They use materials that can embed and detach lithium such as LiCoO2, LiNiO2, and LiMn2O4 as materials for positive electrodes. The electrolyte of lithium-ion batteries generally comprises of organic solvents, electrolyte solids that are lithium salts, and additives. Organic solvents are the main constituent of electrolytes and they significantly affect the property of the electrolyte. Although there are many organic solvents and lithium salts, only a limited few can be used in lithium-ion batteries. Generally the electrolyte comprises a mixture of high dielectric constant solvents, low-viscosity solvents, and solvents that are electrochemically stabile, with wide electrical potential, wide operation temperature ranges, good heat stability, and low toxicity, are chemically stable, safe, and do not chemically react with the current collector and the active ingredients in the batteries.
The electrolyte is an important component of a battery, greatly affecting its properties. In traditional batteries, the electrolyte has an aqueous solution as the solvent. There are many choices for the aqueous solution as many chemicals dissolves well in water and the chemical and physical characteristics of many aqueous solutions are well understood. However, since the theoretical decomposition voltage of water is only 1.23V. Therefore, even when the overpotential of oxygen or hydrogen is taken into account, the maximum voltage is only about 2V for a battery with an electrolyte that comprises of aqueous solutions such as those in lead-acid secondary batteries. Traditional aqueous solutions cannot be used in lithium ion batteries as their voltage can reach as high as 3V to 4V. Therefore, the key to further development of lithium-ion batteries lies in the research of electrolyte solids and organic solvents that do not decompose at high voltage.
A number of problems exist in present lithium ion batteries. A common problem is that the gas generated during charging and discharging from the decomposition of the electrolyte on the surface of the negative electrode increases the thickness of the battery. The electrical conductivity rate of organic electrolytes is also low. In addition, the solvent is flammable and easily evaporates. When the battery is over-charged or short-circuited due to misuse, dangerous conditions such as an explosion can occur. The heat stability of lithium ion batteries poses another safety issue. When the heat generated by the reactions in the battery exceeds its ability to dissipate heat, the temperature of a battery can reach its ignition point, resulting in a fire or an explosion.
In recent years, adding additives is an important research approach to improve the properties of lithium ion batteries. Adding small amounts of certain chemicals, i.e., additives, to the electrolyte of a lithium ion secondary battery, can markedly improve certain properties such as the electrolyte's electrical conductivity rate and the battery's cycling efficiency and reversible capacity. These additives do not significantly increase the production costs of a battery but have the effect of substantially improving its performance.
Many patents have discussed the addition of additives to the electrolyte. Sanyo Company's Japanese Patent 2000-58112, Sony Company's Japanese Patent 2000-156243, and GS Company's Japanese Patent 2001-126765 disclosed that the addition of biphenyl could markedly improve a battery's ability to withstand over-charge. Mitsubishi Company's Japanese patent 2003-77478 disclosed that, by adding 0.02% to 0.1% of biphenyl in the positive electrode, over-charge could be effectively prevented after multiple cycles. Japanese Patent 2001-15155, jointly held by Sanyo Company and UBE, disclosed that, aromatic hydrocarbons such as cyclohexylbenzene are electrochemically active and can effectively prevent over-charge. When a battery is over-charged, excessive amounts of lithium ions escape from the positive electrode and embed in or accumulate on the negative electrode. As a result, the heat stability of the two electrodes is impaired such that the positive electrode becomes prone towards decomposition. The positive electrode would then release oxygen that can catalyze the decomposition of the electrolyte, creating large amounts of heat. The active lithium accumulated on the negative electrode can easily react with the solvent, releasing heat and converting chemical energy into heat energy. The temperature of the battery rises rapidly. As the temperature rises, the electrolyte participates in almost all the reactions within the battery. These reactions include the electrolyte's reactions with the material for the positive electrode, the lithium embedded in the carbon, lithium metal, and the self-decomposition reaction of the electrolyte.
Adding biphenyl, cyclohexylbenzene, or a mixture of both to the electrolyte can prevent over-charge to a certain degree. However, by doing so, the properties of the battery at low temperatures are affected negatively. In addition, the thickness of the battery increases as a result of the large quantity of gas that is generated during the charge and discharge. Even when the additives generate only a small amount of gas, the over-charge and low-temperature properties of the battery may not be good. However, when the additives results in good over-charge properties, the battery may produce large quantities of gas and also may not have good discharge properties at low temperatures.
Due to the limitations of the prior art, it is therefore desirable to have novel additives for electrolytes that can be used in lithium ion batteries and produce batteries with good low temperature properties, over-charge properties, and batteries that generate less gas during charge and discharge.